1. Field of the Invention
The present invention relates to an image reading apparatus that reads an image of an original by a plurality of image sensors arranged in a zigzag manner, in an image scanner, a digital copier, a facsimile machine, and the like.
2. Description of the Related Art
Conventionally, reading devices installed in copiers and facsimile machines for example, and image reading devices such as scanners for inputting information to computers for automatically reading image information of originals are configured to read an image on an original by irradiating the original with light from a light source extended in a direction (main-scanning direction) perpendicular to a conveying path of the original (sub-scanning direction) and receiving reflected light from the irradiated original by an image sensor.
Furthermore, as a conventional method of reading an original, a method is generally used in which an original is irradiated with light from a xenon lamp used as a light source and reflected light from the original is read by an optical sensor via a reducing optical system. However, in recent years, for a purpose of reducing the size of an apparatus, a so-called CIS (Contact Image Sensor) has been put to practical use, in which an LED (Light Emitting Diode) having a small size is used as a light source and a linear sensor is caused to directly read an image via a self-focusing lens for example.
As the linear sensor applied to the CIS, a multichip-type image sensor array has been widely used, which is formed by aligning a plurality of sensor chips (light receiving chips) in each of which a large number of photoelectric conversion sensors is linearly arranged.
However, in the multichip-type image sensor array, because a gap exists between adjacent sensor chips, there is a problem in that an image of an original cannot be read in the gap and image information in the gap cannot be obtained.
To address this problem, a multichip-type image sensor array formed of sensor chips arranged in a zigzag manner may be applied so that the gap between the adjacent sensor chips can be filled by other sensor chips.
However, even when the sensor chips are arranged in a zigzag manner, a space remains between each chip in an even row arranged in the main-scanning direction and each chip in an odd row arranged in the main-scanning direction so as to cover gaps between the chips in the even row, so that image data of an image may be shifted between each chip in the even row and each chip in the odd row in the sub-scanning direction. Therefore, it is necessary to correct a shift amount in the sub-scanning direction. Furthermore, because mounting-accuracy of each chip varies, the correction amount of the shift in the sub-scanning direction may vary depending on each chip.
Moreover, in an apparatus that reads an image by moving an original, a position of the original may be shifted depending on a location because an original conveying speed may vary due to a shock that occurs when the original abuts against a roller that conveys the original, rotation variation of a motor, and the like. In this case, the correction amount of the shift in the sub-scanning direction may vary depending on the position of the original.
To solve the above problems, Japanese Patent Application Laid-open No. 2006-140599 discloses an apparatus that reads linear image patterns along the main-scanning direction and calculates a sub-scanning position where the liner image patterns match each other for each sensor chip, so that it can accurately correct the amount of mounting variation in the sub-scanning direction when the sensor chips are mounted in a zigzag manner.
However, an image reading apparatus disclosed in Japanese Patent Application Laid-open No. 2006-140599 cannot correct a sub-scanning image shift due to the variation in the original conveying speed, although it can correct variation due to the variation in the mounting accuracy of the chips.